Stirling engines

ABSTRACT

A Stirling engine with mechanical output through a crankshaft (57) has piston(s) (42) reciprocable in cylinder bore(s) (41). The piston to crankshaft connection includes a lever arm (74) on pivot pin (77) and a connecting rod (75). The lever arm passes through opening (76) which is sealed from the engine crankcase by an annular seal member (94) engaged against part spherical seat (91) on the lever arm by gas pressure and by a spring washer (65).

The invention relates to Stirling engines. In referring to a Stirlingengine we include those engines which operate on a cycle resembling theStirling cycle but with some overlap and merging of the individualphases of the classical Stirling cycle.

The invention is applicable particularly but not exclusively to Stirlingengines of the multi-cylinder double-acting type. Typical engines ofthis type have a hot working chamber at one end, normally the upper endand a cold working chamber at the other end of each cylinder separatedby the piston, each of these hot and cold working chambers beingconnected respectively to a cold or hot working chamber of anothercylinder. In this way, four closed working volumes are established ineach of which the required working fluid is permanently entrapped.Conventional lubricants can not normally be used within the workingvolume because the lubricant carbonises and carbonised depositsinterfere with heat transfer capability.

This kind of design often incorporates an axial piston rod extendingthrough a sliding seal in the cylinder, running in a cross head bearingand then connected to a shaft, typically through a normal crank drive.Sliding seals of this nature tend to suffer from high friction and wearproblems and wear compensation is difficult to achieve with such a seal.The reduced effective piston area caused by the piston rod can also be adisadvantage.

Alternatives to sliding seals such as rolling diaphragm seals or use ofpressurised crankcases with simple crank mechanisms introduce otherproblems such as unreliability for the sliding seal and excess weight inreducing crankcase volume.

In our co-pending PCT patent application filed simultaneously herewithand claiming priority from our UK application 9008522.6 we describe areciprocatory Stirling engine in which a connection between the pistonand a main shaft comprises a lever arm pivotable intermediate its endsand extending through the cylinder wall, connected at one end thereof tothe piston and at the other end thereof to the main shaft.

An object of the present invention is to provide an effective sealarrangement for such a lever arm.

According to the present invention there is provided a Stirling enginecomprising a drive member, a cylinder, a piston reciprocable in thecylinder, a connection between the piston and the drive membercomprising a lever arm pivotable about a pivot bearing intermediate itsends and extending through the cylinder wall and a gas seal arranged toprevent escape of pressure from the cylinder in the region of the leverarm, the seal comprising a part spherical seat on the lever arm with itscentre coincident with the pivot axis, an annular seal member with apart spherical sealing surface in sealing contact with the partspherical seat and means for urging the seal member into sealingengagement with the seat.

Preferably the pivot bearing comprises a pivot pin passing through thelever arm and mounted at both ends in a pivot housing.

Preferably the connection between the piston and the drive memberdefines a limit of pivotal movement for the lever arm and the width ofthe sealing surface is greater than the movement of a corresponding partof the part spherical seat when the lever arm moves between its limitsof pivotal movement such that there is a specific annular area on thesurface of the seat which is and always remains in engagement with thesealing surface.

Preferably the seal member is carried in a movable seal holder which issupported in a fixed carrier and is urged by gas pressure in the engine,which may be supplemented by a spring, into engagement with the sealseat The annular seal member may be arranged to rotate about its ownaxis to distribute wear evenly around the sealing surface. This rotationmay be derived from movement of the lever arm through a pawl whichengages a ratchet wheel which in turn drives the seal member through areduction gear.

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic cross-section through a four cylinder engine inaccordance with the invention;

FIG. 2 shows part of such an engine in greater detail; and

FIG. 3 shows a modification of part of FIG. 2

FIG. 1 is a diagrammatic cross section through a four cylinder Stirlingengine showing two of its cylinders. The engine layout incorporates twobanks of two cylinders and one cylinder from each bank is shown. Theseare referenced cylinders 2 and 3. The other cylinder in the same bank ascylinder 2 is referred to as cylinder 1 and the other cylinder, in thesame bank as cylinder 3 is referred to as cylinder 4. Cylinder 2 istypical. It has a main bore 11 in which a piston 12 with integraldisplacer 13 reciprocates. The piston incorporates a downwardlyextending piston body tube 14 which surrounds a fixed tubular pistonguide 15 so that the piston 12 is guided on guide 15 rather than by theinternal surface of cylinder 11. The cylinder has a major upper diametercorresponding to the full diameter of the piston and a minor lowerdiameter slightly greater than the diameter of the piston body tube. Agenerally vertical link 18 connects the piston to a lever arm 21 throughpivot pins 16 and 19. The lever arm passes through slots 22 and 23 inthe piston guide 15 and piston body tube 14 respectively.

The lever arm has a fixed pivot 24 and a cranked extension 25. Aconnecting rod 26 interconnects the cranked extension 25 with acrankshaft 27. In this way, reciprocation of piston 12 is connected torotation of the crankshaft 27.

An upper or hot working chamber 28 is provided within the cylinder abovethe displacer 13. The space below the piston 12 is closed off and formsa lower or cold working chamber 29. Each cold working chamber is sealedwith respect to the crankcase 30 so that the crankcase is unpressurisedand parts within it can be lubricated conventionally. The sealingarrangement for arm 21 will be described with reference to FIG. 2.

The mechanical arrangement of a piston 32 for cylinder 3 reciprocable ina cylinder bore 31 is a mirror image of the arrangement for cylinder 2.Piston 32 is connected to crankshaft 27 by a crank pin arranged toprovide a 90° phase difference between the reciprocation of pistons 12and 32.

As illustrated, the cold chamber 29 of cylinder 2 is connected by a gaspassage 34 to the hot chamber 33 of cylinder 3. This connection is madevia a cooler 35 a regenerator 36 and a heater 37 adjacent the hotchamber 33. In practice the heating is provided by combustion gasesducted over the upper part of the cylinder and the cooler uses water asa coolant. Arrows indicate the flow of working fluid between the hot andcold chambers By means of gas passage 34, cold chamber 29 and hotchamber 33 are united into a single closed working volume within whichworking gas operates broadly in accordance with the Stirling enginecycle.

Cylinder 2 is offset axially of the crankshaft 27 to a sufficient extentto allow clearance between adjacent lever arms, connecting rods andcrankshaft connections. The other two cylinders 1 and 4 are arrangedrespectively behind cylinders 2 and 3 and are not shown. They aresimilarly offset slightly from each other and are connected to thecrankshaft via crank pins set at suitable angles to give 90° phaseangles between cylinders 1 and 2, 2 and 3, 3 and 4 and thus also 4and 1. Cylinder 2 is shown at mid-stroke while cylinder 3 is at TDC.There are a total of 4 gas passages corresponding generally to gaspassage 34, each connecting the cold chamber of one cylinder with thehot chamber of an adjacent cylinder. In each case there is acorresponding 90° phase angle between each pair of interconnectedchambers.

This general arrangement of four cylinders, 90° phase angles andinterconnection of hot and cold working chambers is a well known form ofStirling engine layout, known as the Rinia layout so further details ofits operation and will not be described here. FIG. 2 shows details ofone cylinder of an engine similar to that of FIG. 1 but with some detaildifferences in layout.

The cylinder for the engine is constituted primarily by a stainlesssteel cylinder liner 41 the internal face of which provides a surfaceagainst which a piston seal 45 slides. The cylinder extends upward intoa heater head by means of a closed cylindrical stainless steel spinningwhich forms a heater head liner. In use heat is applied continuously tothe heater head so that working fluid is heated and the space above thepiston becomes a hot working chamber Similarly the region below thepiston is cooled continuously, for example by a water cooler surroundingthe liner 41 to provide a cold working chamber below the piston. Furtherdetails of the heating and cooling arrangements may be as in FIG. 1. Theinterior surface of the liner 42 makes no contact with the piston or adisplacer carried on the piston. The liner 41 itself is carried in amain casting 43 which forms an outer cylinder and also forms part of thecrankcase of the engine.

A piston 44 is arranged to reciprocate in the cylinder but makes nodirect contact with the cylinder for guidance. A sliding seal betweenthe piston and cylinder is constituted by a piston ring assembly 45.

The primary structural element of the piston is a cast aluminium alloypiston body tube 46 of substantially greater length than a conventionalpiston. The piston body tube 46 incorporates an upper external flange 47on which is mounted an outer piston body 48 of stainless steel carryingthe piston ring 45 in an external annular groove. The outer piston body48 is secured to flange 47 by interlocking spigots between thesecomponents, a retaining ring 49 and bolts 51 passing through flange 47and retaining ring 49. The retaining ring 49 holds other components inposition and these will be described subsequently.

A fixed cylindrical tubular piston guide 52 extends up into the cylinderin an axial direction. It is secured at its lower end to the crankcaseformed by casting 43 as will now be explained. The piston guide 52incorporates a lower external flange 53 which forms a spigottedconnection to the crankcase and is secured to the crankcase by a ring ofstuds 54. The lower end of the piston guide 52 is closed by anexternally flanged closure member 50 which is secured to the crankcaseby bolts 55, these bolts passing through flange 53 and thus providingfurther fixing for the piston guide 52. Separate sets of bolts 54 and 55are provided so that the piston guide 52 can be installed before theclosure member 50 as an aid to assembly of other parts of the engine.

The piston is guided for sliding movement on the piston guide 52 whichextends up into the piston body tube 46.

The interior of the piston tube body forms a recess which is closed atits upper end as will be described subsequently. The interior surface ofthe piston body tube 46 carries a lower annular bearing pad 56 and alsosupports a bearing pad carrier 57 which carries an upper bearing pad 58.These bearing pads are typically of bronze impregnated PTFE. The pistonguide 52 is typically formed of electroless nickel/PTFE plated mildsteel to provide a bearing surface for the pads 56 and 58 which willoperate satisfactorily in an oil free environment.

The upper bearing pad carrier 57 is secured in a spigot at the upper endof the piston body tube by the retaining ring 49.

The piston 44 also incorporates a displacer crown assembly made up fromstainless steel sheet pressings and spinnings. This is conventionalStirling engine technology so only part of the displacer crown assemblyis shown. The drawing shows part of a dome-topped cylindrical displacercrown 61. A series of full flanged bulkheads 62 and open-centre flangedbulkheads 63 serve to restrict heat transfer from above the displacercrown into the body of the piston and also to stiffen the displacercrown. Blocks of lightweight thermal insulation material may be arrangedbetween and supported by adjacent bulkheads. The displacer crown 61itself is mounted on the outer piston body 48 and is secured by spotwelding.

The upper part of the displacer crown assembly closes the recess in thepiston across the piston diameter above the upper end of the piston bodytube 46 so that the interior of this tube becomes a recess open at itslower end and closed at its upper end.

For the functioning of the Stirling engine, it is desirable that thefree volume below the piston including the volume within the recessreferred to above should be kept to a reasonable minimum For thispurpose, a domed cylindrical internal filler member 64 is mounted on thepiston to form part thereof and extends down inside the piston body tube46. Filler member 64 is a stainless steel spinning and it is mounted inposition by a further spun member 65 which in turn is secured to thepiston body tube 46 by retaining ring 49. Members 64 and 65 also help torestrict heat transfer down through the piston.

As thus far described, piston 44 is freely slidable in an axialdirection in cylinder 41 and is guided to slide on the axially extendingpiston guide 52 by lower and upper bearing pads 56 and 58. This guidemechanism holds the outer surface of the piston clear of the cylinder41.

Piston ring 45 serves only as a sliding seal and not as a guide for thepiston. Because of the laterally unsupported displacer crown well abovethe upper pad 58, a near-constant sliding fit between this pad and thepiston guide is particularly important to piston location.

A crankshaft 71 is mounted in the crankcase formed in main casting 43 torotate about an axis 72 in bearings which are not shown. The crankshafthas a conventional offset crank pin 73. The main componentsinterconnecting the piston and crankshaft are a lever arm 74 and aconnecting rod 75.

Lever arm 74 extends through an opening 76 which is effectively withinthe wall of the cylinder. It is pivotally mounted about a pivot bearingcomprising a pivot pin 77 which is fixed at both ends in a pivot housing78 secured by bolts 79 to the main casing 43. The outer end of lever arm74 is connected by pin 79 to the connecting rod 75 and in this way,crankshaft rotation is coupled to reciprocatory pivotal movement of thelever arm 74.

The inner end of the lever arm 74 extends into the

cylinder 41 and lies substantially on the axis of the cylinder. Toprovide clearance for insertion and reciprocation, the piston body tube46 incorporates a slot 81 and the piston guide 52 incorporates a slot82. Lever arm 74 terminates in an upper piston pivot pin 83 whichconnects the lever arm to a piston link 84 which is forked to providepivot pin anchorages to both sides of the lever arm 74. A lower pistonpivot pin 85 passes through the lower end of the piston link and throughslots 86 in the piston guide 52 to terminate in bores (not shown) in thepiston body tube 46. In this way, the piston 44 is connected forreciprocal movement with the lever arm 74, the link 84 catering for theradial component of movement of the lever arm 74 with respect to thecylinder.

Conventional lubrication can be employed for the crankshaft andconnecting rod bearings and for the pivotal movement of the lever arm 74about pivot pin 77. Lubrication passages can also be provided in thelever arm 74 and link 84 to provide lubrication for pivot pins 83 and85. Alternatively the pivot pins 83 and 85 may employ dry lubricationtechniques.

A gas-tight seal is associated with pivotal movement of the lever arm74. The lever arm itself carries a part-spherical seal seat 91 which ismounted on the lever arm with its centre coincident with the centre ofthe pivot axis of the lever arm. A fixed annular seal carrier 92 ismounted in casting 43 and carries a movable seal holder 93 which in turncarries an annular seal member 94 with a part-spherical surface incontact with the corresponding surface of the seal seat 91. An annularspring 95 which may be in the form of a wavy washer is arranged to urgethe seal holder 93 and the seal member 94 in an outward direction toprovide sealing contact with seat 91. A series of O-rings 96, 97 and 98provide further sealing between components of the seal assembly. Theseal member itself may be of a highly impenetrable grade of PTFE/bronzecomposite, possible alternatives being polyimide resins orPTFE/polyimide mixtures. The seal seat may have a ground stainless steelsurface or it may be electroless plated with PTFE and a metal. A ceramicseal seat is an alternative. The seal is self adjusting in that as weartakes place at the spherical bearing surfaces, the seal member and sealholder are maintained in contact with the seal seat. The seal isarranged to be such that internal pressure within the cylinder acts onthe seal holder both to increase the bearing pressure between the sealmember and the seal seat and to move the seal holder in a direction totake up wear. Effective take up of wear is possible because the movementavailable has a component normal to the wearing surfaces Spring 95establishes initial contact for sealing purposes

FIG. 3 is a scrap view of part of an engine corresponding to that ofFIG. 2 but showing a modification whereby the seal member 94 is causedto rotate slowly in order to even out wear in the seal member. It shouldbe explained that the peripheral speed of the seat 91 is much greater inrelation to the seal member 94 at regions near to the plane of movementof the lever arm 74 than it is at positions 90° around the periphery ofthe seal member.

The mechanism which provides this rotation is as follows. A drive ring105 is mounted in an annular recess machined in the periphery of thepivot housing 78. The drive ring 105 incorporates a ring of ratchetteeth 106 and a single start scroll gear 107 on its outer face. A pawl108 mounted on pivot 109 on pivot pin 77 is held in engagement with theratchet teeth by spring 110.

Reciprocatory movement of lever arm 74 thus indexes the drive ring 105through the distance of one ratchet tooth for each revolution of theengine. Scroll gear 107 in turn drives gear 111 and worm gear 112mounted for rotation therewith. Worm gear 112 drives a further gear 113which has a shaft 114 extending into the pivot housing 78 and also haspinion gear 115 which engages with corresponding external gear teetharound the periphery of the outer seal carrier 92. A recess in the leverarm provides clearance for gear 113 With this modification, the sealholder 93 is engaged with seal carrier 92 in such a manner that both arecaused to rotate together. This engagement may for example be providedby a pin in a keyway.

In use of the engine, movement of the lever arm indexes the drive ring105 which in turn rotates the various gear elements 111, 112, 113 and115 to thereby cause rotation of the seal member 94 derived frommovement of the lever arm. The gearing should be such that one turn ofseal member 94 occurs in several hours of running of the engine. By thismeans, wear of the seal member is evened out to provide it with a longerlife and also to provide more effective sealing.

Another important feature of the seal arrangement as described in bothFIGS. 2 and 3 is as follows. The objective of this feature is to ensurethat no part of the seal seat 91 which is in use exposed to the workinggas in the interior of the engine should at any time be exposed to theregion on the other side of the seal, namely in the engine crankcase.This is achieved by suitable dimensioning of the width A of the seal asshown in FIG. 3 in relation to the limits of pivotal movement of thelever arm. In particular, the width of the sealing surface should begreater than the movement of the corresponding part of the seat 91 whenthe lever arm moves between its limits of pivotal movement. Consideredanother way, there is always a specific annular area on the surface ofthe seat which is and always remains in engagement with the sealingsurface.

The engine shown in FIG. 2 is a double acting four-cylinder Stirlingengine corresponding to the layout shown in FIG. 1. Only one cylinder isshown. In use, the region of the cylinder above the piston is a hotworking chamber and the region of the cylinder below the piston is acold working chamber. This lower region departs somewhat fromcylindrical shape due to the mechanical connection to a piston via thelever arm 74 and due to the mounting of the piston guide. This shapedeparts further from that of a cylinder as such due to the requirementfor reducing the effective volume below the piston to a reasonableminimum when the piston is at its lowermost position. However, pressurebelow the piston acts on the full area of the piston, providing ineffect a full area piston extending across the cylinder and subject topressure.

In use, the working space within the cylinder below the piston isoperated as a cold working chamber in the Stirling engine with theresult that working gas is at a relatively low temperature. This keepsthe temperature of the lower bearing pad 56 low. On the other hand, theupper bearing pad 58 is remote from the main cold working space belowthe piston and could be at an undesirably high temperature due to heattransfer through the piston from the hot working chamber. To reduce thiseffect, cold working fluid is caused to flow past the upper bearing pad.The pad carrier 57 is provided with vents 99 to allow working gas topass through it. The annular volume 100 immediately above the pistonguide 52 and also confined by members 64 and 65 increases and decreasesduring engine reciprocation, causing cold working gas to pass throughthe vents 99. Some gas in volume 100 also enters and leaves through theannular gap between filler member 64 and the interior of piston guide 52but by keeping this gap to a reasonable minimum there is significant gasflow through the vents. This flow of gas tends to hold down thetemperature of the bearing pad carrier 57 and bearing pad 58.

The vents 99 may be made asymmetric s that air flows more easily in onedirection through them than in the other direction. For example, one endmay be provided with a sharp acute angled edge while the other end isprovided with a rounded edge. The result of such an arrangement is a netcirculation of cooling working fluid through the bearing pad carrier 57instead merely of equal alternate flows in both directions.

The arrangement shown allows a compact four-cylinder engine to beproduced. The cylinders are arranged in two parallel banks of twocylinders, one to each side of the crankshaft axis 72. The two banks areoffset in the direction of the crankshaft by a distance equivalent tohalf the pitch between the cylinders in one bank. This allows clearancefor pivot housing 78 and connecting rod 75 between lower minor diameterportions of two cylinders of the other bank, thus allowing the majordiameter portions of the two cylinder banks to be close together andthereby permitting a compact design. Although a relatively long cylinderis required to accommodate the piston body tube and piston guide, thelower part of this cylinder is of reduced diameter which convenientlyprovides clearance for the crankshaft. Thus a compact overall enginedesign can be provided.

In the usual way for a Stirling engine the hot working chamber of onecylinder is in continuous connection through heating and coolingfacilities with the cold working chamber of another cylinder which isoperating at an appropriate phase angle to the first mentioned cylinder.

As a departure from the four cylinder double-acting layout, two singleacting cylinders could be employed.

A further alternative would be a single cylinder arrangement with asupplementary lower piston co-axial with the main piston. Thesupplementary piston should be connected to the crankshaft at such aphase angle as to provide the required relationship between expansionand contraction of the hot and cold working chambers so that thechambers from the same cylinder can be interconnected to provide aStirling engine.

I claim:
 1. A Stirling Engine comprising a housing, a drive member, acylinder defined within the housing whereby the housing defines acylinder wall, an aperture defined in the cylinder wall, a pistonmounted for reciprocation in the cylinder, a pivot bearing having apivot axis fixed with respect to the housing, a connection between thepiston and the drive member comprising a lever arm having two ends andbeing pivotally mounted about the pivot bearing intermediate the ends ofthe lever arm and extending through the aperture, and a gas sealarranged to prevent escape of pressure from the cylinder through theaperture around the lever arm, the seal comprising a part spherical seaton the lever arm with its centre coincident with the pivot axis, anannular seal member sealingly mounted with respect to the housing andhaving a part spherical sealing surface in sealing contact with the partspherical seat and means for urging the seal member into sealingengagement with the seat.
 2. A Stirling Engine as claimed in claim 1wherein the pivot bearing comprises a pivot housing fixed with respectto said housing and a pivot pin passing through the lever arm andmounted at both ends thereof in said pivot housing.
 3. A Stirling Engineas claimed in claim 1 comprising limits of pivotal movement for thelever arm defined by the connection between the piston and the drivemember, the sealing surface having a width which is greater than adistance defined by movement of a corresponding part of the sphericalseat when the lever arm moves between its limits of pivotal movementsuch that there is a specific annular area on the surface of the seatwhich is and always remains in engagement with the sealing surface.
 4. AStirling Engine as claimed in claim 1 further comprising a movable sealholder within which the seal member is carried.
 5. A Stirling Engine asclaimed in claim 3 further comprising a fixed seal carrier within whichthe movable seal holder is supported and wherein the seal carrier isurged by gas pressure in the engine into engagement with the seal seat.6. A Stirling Engine as claimed in claim 5 further comprising a springarranged to urge the seal carrier into engagement with the seal.
 7. AStirling Engine as claimed in claim 1 wherein the engine is adouble-acting Stirling Engine having a hot working chamber and a coldworking chamber and wherein the lever arm extends through the cylinderwall into the cold working chamber thereof.
 8. A Stirling Engine asclaimed in claim 1 further comprising means to rotate the annular sealmember bout its own axis to distribute wear evenly around the sealingsurface.
 9. A Stirling Engine as claimed in claim 8 comprising drivemeans for seal member rotation operatively connected to the lever armand to the seal member to rotate the seal member in response to movementof the lever arm.
 10. A Stirling Engine as claimed in claim 9 furthercomprising a pawl, a ratchet wheel and a reduction gear, wherein thepawl is operated by movement of the lever arm and engages the ratchetwheel which in turn drives the seal member through the reduction gear.